Classification

Distribution

This species is widely-distributed being known from much of the Amazon basin from the Ríos Marañon and Ucayali in the west (Peru) to the rios Purus, Madeira, Branco, Amazonas, Trombetas, Xingu, and Tocantins in the east (Brazil.

Three specimens collected in the rio Pindaré, Maranhão state, Brazil represent the only known record of a cynodontid species in northeastern Brazil.

It also occurs in middle and lower portions of the Río Orinoco basin, Venezuela and the Rupununi River in the upper Essequibo basin, Guyana.

Type locality was orginally given as ‘Brazilian rivers’ by Agassiz but the specimens were apparently destroyed or lost so a neotype with locality ‘Lago Manacapuru, Amazonas State, Brazil’ was designated by Toledo-Piza (2000).

Habitat

This species is pelagic and adults tend to be associated with main river channels and larger tributaries of both white and black water rivers.

It occurs sympatrically with the congener C. septenarius in the rio Uatumã, Brazil and the two also occur in proximity in the rio Branco and rio Trombetas systems.

Maximum Standard Length

The largest officially-recorded specimen measured 280 mm but it may grow larger.

An aquarium with base dimensions of 210 ∗ 75 cm should be the absolute minimum considered.

Maintenance

The aquarium should ideally be designed to resemble a flowing stream or river with a substrate of variably-sized rocks, sand, fine gravel, and some larger water-worn boulders.

This can be further furnished with driftwood roots and branches if you wish but be sure to leave plenty of open swimming space.

Like many fishes that naturally inhabit running waters it’s intolerant to the accumulation of organic wastes and requires spotless water at all times in order to thrive.

It also does best if there is a high proportion of dissolved oxygen and moderate degree of water movement so external filters, powerheads, airstones, etc., should be employed as necessary.

As stable water conditions are obligatory for its well-being this fish should never be added to biologically-immature aquaria and weekly water changes of 30-50% aquarium volume should be considered mandatory.

A tightly-fitting cover is also essential as it’s likely to prove a prodigious jumper.

Water Conditions

Diet

Newly-imported specimens often refuse to accept anything but live fishes but most can be weaned onto dead alternatives once recognised as edible.

Like the vast majority of predatory fishes this species should not be fed mammalian or avian meat like beef heart or chicken, and similarly there is no benefit in the long-term use of ‘feeder’ fish such as livebearers or small goldfish which carry with them the risk of parasite or disease introduction and at any rate tend not have a high nutritional value unless properly conditioned beforehand.

This species is rare in the aquarium trade but is arguably more suitable for the home aquarium than some of its better-known relatives given its adult size and relatively docile behaviour.

Other vernacular names include ‘Dientón’ (Peru), ‘Perrito’ (Ecuador), ‘Payara-chata’ or ‘Payarin’ (Venezuela) and ‘Icanga’, ‘Minguilista’ or ‘Peice-cachorro’ (Brazil), some of which are also applied to related species.

There currently exist just two other species in the genus and C. gibbus can be distinguished from C. septenarius by possessing (vs. lacking) a dark band at the base of the caudal-fin rays, 8 (vs. 7) branched pelvic-fin rays and orbital diameter measuring 24.9–33.8 % HL (vs. 30.1–34.4 %).

C. meionactis also appears restricted to the upper Maroni river in French Guiana and Suriname.

Cynodoncan be told apart from other genera in the family Cynodontidae by a number of synapomorphies plus the following external characters: anal-fin relatively long with 60+ branched rays (vs. <50 in Rhaphiodon and Hydrolycus); anal-fin origin located approximately at a vertical through the middle of the body length and almost reaching the tips of the pelvic-fin rays (vs. anal-fin origin distinctly posterior to the middle of the body).

It’s sometimes included in the putative subfamily Cynodontinae alongside Rhaphiodon and Hydrolycus, these being separated from other Characiformes by a series of derived features plus their oblique mouth shape and highly-developed dentary canine teeth.

The latter fit into a pair of corresponding openings in the upper jaw which allows the mouth to be closed completely.

Cynodontinae contains two primary monophyletic lineages, one comprising the genus Hydrolycus and the other a clade with Cynodon and Rhaphiodon spp., with members sometimes referred to collectively as ‘dogtooth characins’.

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